A comparison of the fragmentation of different species of mammalian immunoglobulin M by trypsin in urea.

Six different species of mammalian IgM that had been preincubated in 4-6 M urea showed marked differences in their fragmentation by trypsin at 25 degrees C. These differences are most probably due to different degrees of conformational change in urea particularly as regards the C mu 2 domains. The fragments obtained by digestion with trypsin in urea were often different to those obtained by digestion in aqueous buffer at 37 or 55 degrees C. In particular the production of (Fc*)5 fragment containing C mu 2 domains was favoured. In the case of human and mouse IgM the fragmentation could be controlled sufficiently to produce series of molecules which differed in their numbers of Fab arms and/or C mu 2 domains.